Infrared determination of orientation, orientation distribution, and crystallinity in polyethylene films

Abstract

AbstractThe problem of determining orientation distribution in polymer films from the measurement of infrared dichroism is treated from the theoretical point of view. It is shown that the distribution function cannot be determined from such measurements. However, if an elliptical distribution function is assumed, the axial ratio of the ellipse of planar components may be calculated. The curve for axial ratio as a function of measured dichroic ratio is given. The dichroisms of the 1300, 730, and 719 cm.−1 bands in polyethylene are discussed and interpreted. It is shown that, in extruded films, chain axes in the amorphous areas lie roughly in the extrusion direction. Chain axes in the crystalline areas lie roughly perpendicular to the extrusion direction. In stretched films, all chain axes line up in the stretch direction. The axial ratios calculated from the 730 and 719 cm.−1 bands are given for a cold‐stretched polyethylene film at different degrees of stretch. Axial ratios of over 100:1 may be attained for the crystalline regions of the film. Specific extinction coefficients for the crystalline band at 730 cm.−1 and the amorphous band at 1300 cm.−1 are determined from measurements on polyethylene films of different crystallinities. The latter values is compared with values measured in molten polyethylenes and normal hydrocarbons. The hydrocarbon values do not agree with the polyethylene values. The amorphous contents of a number of polyethylenes as measured by infrared, density, and x‐ray methods are given. Possible reasons for the disagreement between the results are briefly discussed.